Why Excel changes dates (and other values) when you open a CSV
Why dates, gene/product codes, and other text values sometimes silently change when a CSV is opened in Excel, and how to stop it from happening.
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Why this happens
Same root cause as our leading-zeros issue: Excel guesses each column's type when it opens a CSV, and it's quick to guess "date" for anything that looks date-like. A product code like "2-1" or "MAR1" can silently become a date (e.g. "2-Jan" or a serial date number) the moment the file is opened — and if you save over the original file, that change is now permanent in the data itself, not just in the display.
Preventing it on import
Same fix as leading zeros: use Data → From Text/CSV instead of double-clicking the file, and explicitly set the affected column(s) to "Text" during import so Excel doesn't reinterpret them.
If dates need to stay dates, but in a specific format
When a column genuinely is dates and you need it to display a specific way (e.g. ISO YYYY-MM-DD) after import, set that column's import type to the date format matching your source data, rather than "General" — letting Excel auto-detect the format is what causes regional mismatches (month/day swapped) as well as the type-guessing problem above.
The safest habit: never open the original export directly
Whichever column types matter, the safest overall habit is the same one from our other guides: keep the original CSV export untouched, and do all cleanup/import work on a copy. If Excel silently mangles a copy, you can always re-export or re-copy from the untouched original.
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